Modified polytetrafluoroethylene compositions and products related thereto



Feb. 19, 1957 v. w. WEIDMAN 2,782,180

MODIFIED POLYTETRAFLUOROETHYLENE COMPOSITION AND PRODUCTS RELATEDTHERETO Filed Aug. 25, 1952 of pressure above L000 0 sn formirlg shopedorlicles Si nler of temperature above INVENTOR Verne Wesley Weidmun BY IATTORNEY United States Patent MODIFIED POLYTETRAFLUOROETHYLENECOlVlPOSITIONS AND PRODUCTS RELAT- ED THERETO Verne Wesley Weidman,Wilmington, Del., assignor to E. I. du Pont de Nemours andv Company,Wilmington, Del., a corporation of Delaware Application August 25, 1952,Serial No. 306,301

8 Claims. (Cl. 260-41) This invention concerns modifiedpolytetrafluoroethyl Mixtures of carbon with polytetrafluoroethylenehave been known heretofore. Such mixtures have been molded underpressure to form shaped articles in which modulus of rupture has beenrelatively low.

An object of this invention is to provide improved compositions based onpolytetrafluoroethylene', or closely related polymers, and solidfillers. An additional object is to provide a new and usefulpolytetrafluoroethylene composition in the form of a dry, molding powderhaving a high content of solid filler, which composition, nevertheless,yields shaped articles having a relatively high modulus of rupture.Other objects will be apparent from the description of the inventiongiven below.

The present invention provides a pressure-coalescing compositioncomprising colloidal particles of a polymer of the class consisting oftetrafluoroethylene polymer and chlorotrifluoroethylene polymer, finelydivided fill-er, such as a filler from the group consisting of finelydivided carbon (including graphite, etc.), zircon, titania, or the like.

polymer. A preferred filler which may be employed in the practice of theinvention is coke flour. In general the preferred weight ratio of fillerto the said polymer is about 1:1 to 9:1. The invention also comprisescertam processes for the preparation of the said composi-' tions, as setforth below. Whereas the invention will be described chiefly Withrespect to polytetrafluoroethylene (i. e., tetrafiuoroethyl Thepolymeric ingredient may be a mixture of tetrafluoroethylene polymer andchlorotrifluoroethylene 2,782,180 Patented Feb. 19, 1957 ice methanoland ethanol) which are present during the.

polymerization reaction. These polymers are hereinafter referred to astelomers. The term tetrafluoroethylene polymer as used herein covers theabove three types of polymers, it being understood that all thetetrafiuoroethylene polymers of this invention possess a high degree ofpolymerization and a sintering temperature of at least 300 C. Above thesintering temperature such polymers form a gel but they do not actuallymelt to a liquid. This is in contrast to the known relatively lowmolecular weight polymers derivedfrom tetrafluoroethylene and certaintetrafluoroethylene polymer waxes, both of which have sharp meltingpoints.

Methanol-tetrafluoroethylene telomers are tetrafluoroethylene polymerswhich are prepared in the presence of methanol and which containmethanol combined chemically with the said tetrafluoroethylene polymer;it is believed that the molecules of methanol-tetrafluoroethylenetelomer contains end groups supplied by the methanol, so that theformula may be written In certain compositions employed in the practiceof this invention the number of tetrafluoroethylene units in thetelomer'is sufficiently high so that the sintering temperatime (i. e.the minimum sintering temperature) is about 327 C.; accordingly, in theformula just given, x is quite high, usually in excess of 100.

The methanol-tetrafluoroethylene telomers which are employed in thepractice of this invention may be prepared by polymerizingtetrafluoroethylene in the presence of methanol and a free radicalproducing catalyst such as an organic peroxide. Usually thepolymerization is conducted at a temperature within the range of about 0to 100 C.', telomers having relatively lower molecular weights beingformed at higher temperatures. In certain embodiments, thepolymerization may be conducted under superatmospheric pressure. Inorder to obtain the methanol-tetrafiuoroethylene telomers in colloidalform it is generally desirable to carry out the polymerization inaqueous methanol containing a watersoluble polymerization catalyst andan alkali metal or ammonium salt of an acid of the formula ethylene inthe presence of methanol and anorganic peroxide catalyst at atemperature between and 3 350 C., as disclosed in U. S. patentapplication S. N. 65,063, filed by R. M. Joyce on December 13, 1948.Such salts of fluoroalkanoic acid serve as dispersing aids. It is to beunderstood, however, that other dispersing aids may be employed in placeof the salt of the abovementioned fiuoroalkanoic acid.

Any suitable method for admixing the colloidal particles oftetrafluoroethylene polymer with the finely divided, solid filler may beemployed. For example, the mixture obtained in the polymerization,containing the polymer dispersed in a liquid phase can be admixed withthe finely divided filler, and the mixture can thereafter be dried in anoven to remove all of the water. Alternatively, the colloidal particlesof the polymer can be suspended in water containing about 0.25 topreferably 0.25 to 2.5% (based on the weight of polymer), of adispersing agent such as a sodium alkyl sulfate, in which dispersion thefinely divided filler can be dispersed; upon drying the resultingmixture a suitable molding composition can be obtained. In general it ishighly important to remove all volatile matter from the moldingcomposition prior to producing shaped articles therefrom. The drying canbe accomplished at any temperature up to the sintering temperature; asuitable drying temperature is about 100 to 110 C.

The compositions obtained in the practice of this invention can bemolded by compression, preferably through the use of pressures exceedingone thousand pounds per square inch. Relatively lower pressures aresufiicient for hot pressing, and relatively high pressures (up to 10,000pounds per square inch and higher) may be used with advantage for coldpressing.

One of the important aspects of a specific embodiment of the inventionresides in providing a composition which undergoes no change indimensions or virtually none, upon sintering. This is highly significantbecause, after the herein-disclosed compositions have been molded bycompression, it is necessary to heat the shaped articles thus formed toa sintering temperature in order to produce articles having a highmodulus of rupture. A sintering temperature above 300 C. (preferablyabout 327 to 410 C.) is used for this purpose. The minimum sinteringtemperature is 300 to 327 C. At temperatures in the range of 450 C. to500 C., and higher, partial degradation of the polymer may occur, asevidenced by the liberation of a substance which etches glass.

Compositions in which the weight ratio of tetrafluoroethylene polymer tocoke flour is a critical value within the range of 1:2 to 1:3 undergo nochange in dimensions or virtually none, on sintering. The proportion ofpolymerzfiller required to avoid dimensional change differs with thenature of the filler; with graphite as filler the ratio is about 1:1;with zircon it is about 1:4- to 1:6. In other polytetrailuoroethylenemolding composition a dimension change generally does occur onsintering, and this is undesirable from the standpoint of quality andappearance of the product, especially when the production of largepieces is involved.

The time required for heating the molded products at the sinteringtemperature varies with the size of the shaped article; for ordinarysmall articles not exceeding a few inches in largest dimension, asintering time of about 0.5 to 1.0 hour is sufiicient, but forrelatively large pieces a longer sintering time should be employed.

The invention is illustrated further by means of the accompanyingdrawing and the following examples.

Example 1.-Two parts by weight of coke flour and one part by weight ofmethanol-tetrafluoroethylene telomer in the form of a stabilized (1%Duponol, sodium alkyl sulfate) aqueous suspension of colloidal particles(37%, by weight) were intimately admixed and the resulting suspensionwas freed directly of water by evaporation at 110 C. The resultingmixture was molded into a plate by compression at a pressure of 10,000lbs/sq. in. at room temperature. The plate thus obtained was sintered byheating at 400 C. for 0.5 to 1.0 hour. The sintered plate after coolinghad a modulus of rupture of 1400 to 1700 lbs/sq. in. as measured by testC6744, A. S. T. M. Standards 1946, Part II, Nonmetallic Materials (sizeof test specimen 2 x 0.5 x 0.25 in., with a span between supports of 1.5in.). The corresponding modulus of rupture for a similarly preparedcomposition employing colloidal polytetrafluoroethylene particlesobtained without using the methanol modifier was 900 to 1000 lbs/sq. in.A similarly prepared plate employing finely divided granularpolytetrafiuoroethylene of particle size somewhat larger than colloidalhad virtually no strength, and could be crumbled readily by hand.

Example 2.--Example 1 was repeated using an aqueous slurry of cokeflour, blended with an unstabilized suspension of themethanol-tetrafiuoroethylene tclomer, and coagulated by gentleagitation. The water phase was separated from the solid phase, and thesolids were ovendried as in Example 1. The modulus of rupture of themolded product was 1400 to 1700 lbs./ sq. in.

Example 3.Example 2 was repeated, varying, however, the ratio of carbonto methanol-tetrafiuorocthylenc telomer. The results are set forth inthe following table:

Ratio of C/telomer to 400 to 500 1,400 to l, 700

Example 4.-One part by weight of finely divided graphite in the form ofan aqueous slurry and two parts of methanol-tetrafiuoroethylene telomersuspension (stabilized by about 1% sodium alkyl sulfate stabilizer as inExample 1) were intimately admixed and moldings were prepared from theresulting blend as in Example 1. The modulus of rupture of the platesthus obtained was 1550 to 1800 lbs./sq. in.

Example 5.-Example 4 was repeated using a 1: 1 weight ratio of graphiteto methanol-tetrafiuoroethylene telomer. The modulus of rupture wasagain 1550 to 1800 lbs/sq. in.

Example 6. xample 4 was repeated using a 2:1 weight ratio of graphite tomethanol-tetrafiuoroethylene telomer. The modulus of rupture was again1550 to 1800 lbs/sq. in.

Example 7.One part by weight of methanol-tetrafluoroethylene telomer inwater suspension and various parts by weight of finely divided zircon(set forth below) were blended by admixing an aqueous slurry of thezircon with unstabilized methanol-tetrafiuoroethylene telomer ofcolloidal size. The mixtures were coagulated by addition of acetone,filtered and oven-dried at 110 C. The resulting compositions were moldedunder pressure, and a modulus of rupture was determined, with results asfollows:

Ratio of Modulus of Molding Pressure (lbs/sq. in.) Zircon: rupture (lbs!Telomer sq. in.)

10,000 4/1 1, 200 to 1. 350 10,000 2/1 2, 000 to 2, 150

TABLE I [Polymer/embers:1/2.

, Modulus Test Colloidal-sized Polymer Stabilizer 3 Remarks of Rup- No.ture (lbs./

sq. in.)

1 Methanoltetrafluoroethyl- 1% Duponol- Dry coke flour added to 1,325

ene telomer. 37% disp.

n-- 1,530 Dry coke flour added to 1,212

37% disp. (big batch). 1 coke flour: 1 water (slur- 1, 412

17) added to 37% disp. (stirred with spatula). 5 (in dn 1 coke flour: 2Water 1,537

(slurry) added to 37% disp. (agitated gently). 6 do do 1 coke flour: 9water 1,313

(slurry) added to 37% disp. (agitated gently). P 7 An dn 1 coke flour: 1Water 1, 698

(slurry) added to 37% disp. (agitated gently). R fin dn 1 coke flour: 2water 1, 438

(slurry) added to 37% disp. (agitated gently).

9 Tetrafiuoroethylene, not 1% Duponol. Dry coke flour added to 964methanol-modified. 37% disp.

10 (in None (in 2,260

11- Methanoltetrafiuoroethyldo Same mixing 8S8 138 ene telomer 1Telomer: 4 carbon.

12 (in do (In 461 1 Except for batches 11 and 12. i The stabilizer isadded to keep the polymer in dispersion during shipment and storage, andit does not necessarily produce any important effect on the finalproduct. I

TABLE II [Polymer/graphite=1/2.]

Modulus Test Colloidal-sized Polymer Stabilizer Remarks of Rup- No. ture(lbs./

sq. in.)

13 Methanoltetrafluoroethyl- None 1 graphite No. 38: 3% 1,682 enetelomer. water (slurry) added to 37% disp. 14 do do (in 1,621 15 do 1%Duponol 1 graphite N0. 38:4 water 1,588

(slurry) added to 37% disp. (big batch).

TABLE III [Polymer/zircon=l/3.33]

Modulus Test Colloidal-sized Polymer Stabilizer Remarks of Rup- No. ture(ibs./

sq. in.)

Methanoltetrafluoroethyl- 1%Dupon0l Dry filler added to dis- 1,370

ene telomer. persed polymer Polytetrafluoroethylene, do do 1,060

not methanol modified. 18 Methanoltetrafluoroethyl- None -d0 1,520

ene telomer.

Example 9.A series of corrosion tests was made using ture, and sinteredas hereinabove described. In each 1-inch squares of filledmethanol-tetrafluoroethylene instance the squares were immersed in theboiling retelomer compositions stabilized with 1% Dup0no1,telomer agentsset forth in the following table, with results given molded at 10,000lbs./ sq. in. pressure at room temperabelow.

TABLE IV Filled telomer compositions corrosion resistance determinationsWeight Loss, Percent Reagent 1 Teleomer: 2 Carbon 1 Telomer: 2 Graphite1 Telorner: 4 Zircon 120 hr. 192 hr. 360 hr. 120 hr. 192 hr. 360 hr. 120hr. 192 hr. 300 hr.

4. 96 13. 2 3. 79 13. 3 13. 0 30. 3 +0.13 +0. 11 +0. 02 +0. 02 +0.02+0.15 +0.13 +0. 10 10% NaOH +0. 19 +0. 06 +0. 07 +0. 05 0. 05 0. 06 0.02 2. 98

1 Discontinued-swollen and cracked.

Example 10.-A series of tests was made wherein tetrafiuoroethylenepolymer particles of colloidal size were admixed with filler. The drymixture was shaped ina mold by compression under 10,000 pounds persquare inch pressure at room temperature to form specimens which were2-inch square blocks, A inch thick, approximately. The specimens weremeasured carefully at room temperature after pressing; they were thensintere'd'in an oven at 395 C. for 40 minutes, after which they werecooled to room temperature (25 C.) and measured again. The followingtable shows the dimensional stability i. e. the constancy of the size ofthe block, as measured at room temperature), of several specimens, ascompared with the dimensional instability of others. The factorsaffecting dimensional stability are also set forth in the table.

TABLE V Modulusof Rupture 2, 900 After 2 WkS, in-

Percent Wt. Loss After Immersion at Reagent mWlneh Specimen RefluxTemperature lor- Wos Immersed 48 hrs. 120 hrs. 102 hrs. 330 hrs.

20% nor +0. 03 0.0 0. +0. 05 0. 0 1.33 1. 45 1.18 +0. 67 o. as 0.89 r o.as

filler-polymer compositions Weight Length of Length of Nature MethodRatio of Molded Molded Dimension of Nature of Filler of Polymer: BlockPrior Block After Change Polymer Mixing Filler t0 Sinter- Sintering(in.)

ing (in.) (in.)

A Coke flour M 1:1 2. 014 1. 984 020 A- -d0 M 1:2 2. 024 2.010 -.0l4 11.d0 M 1:3 2. 036 2. 040 004 B- --d0 N 1:1 2. 020 1. 995 025 B- do N 1:22. 036 2.016 020 B -do N 1:3 2.032 2. 024 006 B. Powdered graphite. N1:1 2. 015 L995 020' B, .d0 N 1:2 2.018 2. 034, 016 B do N 1:3 2. 019 2.052 033 B- Powdered Zircon... N 1:2 2.009 1. 980 029 B N 1:4 2. 008 1.997 -.011 B -do N 1:6 2.006 2.005 .001'

A. Tetrafiuoroethylenc homopolymer of colloidal size, without dispersingagent, suspended in water.

B. Same as A, but dispersed in ammoniacal aqueous solution containingdispering agent Since dimension change is inversely related to densitychange, it is apparent from the foregoing table that the density changeon sintcring (densities being compared at 25 C.) can be limited to a fewhundredths of a. unit or less by employing the preferred critical weightratio of polymenfiller. This critical ratio difiers with the nature ofthe filler, each filler having its own characteristic critical ratio forno dimensional change. When the amount of filler exceeds the criticalamount, a volume increase on sintering is generally observed; when theamount of filler is less than the critical value, a volume decreaseoccurs on sintering. In the table given above a dimension change of0.007 in. is equal to a density change of 0.01 unit.

Example 11.A (by weight) colloidal dispersion ofchlorotrifiuoro'ethylene polymer in water was mixed with coke'flour(weight proportions, 1 polymerzl coke flour) and thinned with toluenesufficiently to produce a slurry whichcould be'filteredl The powderymixture obtained by filtration was dried, which yielded a free-flow ingpowder. The dry powder was shaped in a mold at ordinary temperature C.)under a pressure of 10,000 pounds per square inch. The preforms thusobtained were removed from the mold and were sintered by heating in anoven for minutes at 345 C. The sintered articles had the followingproperties.

filler mixed with aqueous suspension of polymer-l-mixturc dried at C.

The chlorotrifluoroethylene polymers referred to hereinabove haveproperties similar to those of polytetrafiuoroethylene. Molded,sintered, objects made from colloidal-sized polymerizedchlorotrifluoroethylene dr'y particles and finely divided carbon aresimilar to those made from polytetrafluoroethylene and finely dividedcarbon. For example, when the weight ratio of filler to polymer is from2:1 to 3 :1 the shaped articles obtained from eitherpolytetrafluoroethylene or polychlorotrifluoroethylene, by compressionin a mold (e. g. at room temperature) under pressure in excess of 1000pounds per square inch, followed by sintering at 300 to 410 C., havecompressive strengths above 5000 pounds per square inch and generallywithin the range of 5000 to 10,000 pounds per square inch. Likepolytetrafiuoro ethylene, polychlorotrifiuoroethylene has a minimumsintering temperature of 300 to 327 C. The fillers which are effectiveare the same as in the case of polytetrafiuoroethylene, viz. zircon,graphite, coke flour, etc.

It will be understood that the above examples are illustrative only andthat numerous embodiments of the invention will occur to those who areskilled in the art. For example, various forms of finely divided carbonmay be employed. The particle size of the carbon can be varied. Forexample, commercially available graphite powder in such a state ofsubdivision that 99% passes through a 200 mesh screen gives excellentresults, but graphite in the form of somewhat larger particles also ishighly satisfactory. Similarly a commercially available coke flour insuch a state of subdivision that 56% passes through a 200 mesh screen,the remainder particles passing through a 35 mesh screen, givesoutstanding results, but coke flour of larger particle size is alsoeffective.

As illustrated hereinabove, high quality materials of construction canbe obtained from mixtures containing as high as 90%, or more, of carbon,in accordance with this invention.

This invention is useful in the manufacture of chemically inertmaterials of construction, such as pipes, tubes, pipe liners, bushings,seals, reaction vessels, valves, crucibles, and the like, as well asvarious specialties such as electrical insulators, decorative articles,etc. It is also useful in the manufacture of shaped articles by methodsother than by pressure molding, e. g. extrusion, casting, puddling,injection molding, and calendering I claim:

1. A pressure-coalescing molding composition comprising colloidalparticles of a polymer of the class consisting of tetrafluoroethylenepolymers and chlorotrifluoroethylene polymers, intimately admixed withcoke flour filler, the quantity of filler being from 1 to 9 parts byweight per part of said polymer, the minimum sintering temperature ofthe polymer being within the range of 300 to 327 C.

2. A pressure-coalescing molding composition characterized in thatmolded articles obtained therefrom under a pressure in excess of 1000pounds per square inch do not change dimensionally upon being heated toa sintering temperature within the range of 300 to 410 C., saidcomposition comprising a tetrafluoroethylene polymer in the form ofparticles of colloidal size intimately admixed with coke flour, theweight ratio of coke flour to the said polymer being 1:1 to 3:1, theminimum sintering temperature of the polymer being within the range of300 to 327 C.

3. The composition of claim 2 in which the said polymer is atetrafluoroethylene homopolymer.

4. The composition of claim 2 in which the said polymer is amethanol-tetrafluoroethylene telomer.

5. A pressure-coalescing molding composition coniprising atetrafluoroethylene polymer and coke flour, said composition beingcharacterized in that molded articles obtained therefrom by shapingunder a pressure in ex cess of 1000 pounds per square inch do notundergo a density change, measured at 25 C., of greater than 0.01 whenheated to a sintering temperature within the range of 300 to 410 C.,said composition comprising a tetrafluoroethylene polymer particles ofcolloidal size intimately admixed with coke flour, the weight ratio ofcoke flour to the said polymer being within the range of 2:1 to 3:1, theminimum sintering temperature of the polymer being within the range of300 to 327 C.

6. The composition of claim 5 in which the said polymer is atetrafluoroethylene homopolymer.

7. The composition of claim 5 in which the said polymer is amethanol-tetrafluoroethylene telomer.

8. A pressure-coalescing molding composition comprising a polymer of theclass consisting of tetrafluoroethylene polymers andchlorotrifluoroethylene polymers admixed with coke flour, saidcomposition being characterized in that molded articles can be formedtherefrom by shaping under a pressure in excess of 1000 lbs. per squareinch followed by heating at a sintering temperature within the range of300 to 410 C., have compressive strengths above 5000 pounds per squareinch, said composition comprising the said polymer particles ofcolloidal size admixed with coke flour, the weight ratio of coke flourto the said polymer being within the range of 2:1 to 3:1, the minimumsintering temperature of the polymer being within the range of 300 to327 C.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PRESSURE-COALESCING MOLDING COMPOSITING COMPRISING COLLOIDALPARTICLES OF A POLYMER OF THE CLASS CONSISTING OF TETRAFLUOROETHYLENEPOLYMERS AND CHLOROTRIFLUOROETHYLENE POLYMERS, INTIMATELY ADMIXED WITHCOKE FLOUR FILLER, THE QUANTITY OF FILLER BEING FROM 1 TO 9 PARTS BYWEIGHT PER PART OF SAID POLYMER, THE MINIMUM SINTERING TEMPERATURE OFTHE POLYMER BEING WITHIN THE RANGE OF 300* TO 3 27*C.